The present invention relates to electromagnetic rotary machines and more particularly to an electromagnetic machine such as an induction motor or an induction generator wherein a stator is provided with a position control winding to form a magnetic bearing.
There is an increasing need for a high speed, high power motor in a machine tool, a turbo molecular pump, and a flywheel. In order to rotate the motor at high speeds and eliminate the need for maintenance for a long period of time, there is a tendency for the motor to include a magnetic bearing.
FIG. 18 shows a high speed motor with a plurality of magnetic bearings of a five-shaft control type.
A high speed motor 10 includes two radial magnetic bearings 14 and 16 and a thrust bearing 18. A rotor 12 is radially supported by the two radial magnetic bearings 14 and 16 and axially supported by the thrust bearing 18 while the rotor 12 is maintained out of contact. A three-phase inverter 20 is adapted to actuate a motor 22 so as to rotate the rotor. Each of the magnetic bearings 14, 16 and 18 includes an electromagnet so as to produce a radial or axial magnetic force. An exciting current flows through each of the electromagnets under the control of three single-phase inverters 24, 26 and 28. This enables positional control of the rotor 12.
The size of the radial bearings 14 and 16 tends to be larger in order to produce sufficient magnetic force to support the rotor 12. This results in an increase in the axial length of the rotor 12 and thus, creates a problem that a resilient vibration occurs when the rotor is rotated at high speeds. To provide high power, it is necessary to further increase the axial length of the rotor 12. This brings about an increase in the extent of suction of the motor. To this end, it is necessary to increase the size of the magnetic bearings. This lowers the critical speed of the motor and makes it difficult to rotate the motor at higher speeds.
Accordingly, there has recently been proposed an electromagnetic rotary machine wherein a motor stator is provided with magnetic bearing position control windings so as to reduce its axial length and produce high power.
FIG. 19 shows an electromagnetic rotary machine with associated position control windings.
An electromagnetic rotary machine 30 includes a torque winding (not shown) connected to a three-phase inverter 20 and radial position control windings (not shown) wound around motor stators 32 and 34 and adapted to apply a radial magnetic force to a rotor 31. An electric current flows through the radial position control windings under the control of respective three-phase inverters 36 and 38. This control allows for radial positional adjustment of the rotor 31.
With the electromagnetic rotary machine 30, a single rotor is designed to produce both torque and radial force. This permits a reduction in the axial length of the rotor. Also, the electromagnetic rotary machine 30 produces more power than the electromagnetic rotary machine 10 shown in FIG. 18, provided that they have the same axial length.
There have also been proposed several other electromagnetic rotary machines with associated position control windings.
One arrangement is such that a change in exiting magnetic flux produces an axial force. This force is used to adjust the axial position of a disk-shaped motor. This arrangement is applicable to a disk-shaped rotary machine, but not to a popular radial-type rotary machine.
Another arrangement is such that with a typical induction motor, an imbalanced current is caused to flow through a winding so as to produce a radial force. This force is used to adjust radial position of a rotor. However, this arrangement presents a problem that on principle, a radial force can not be produced when the rotor is located at the center.
As disclosed in Japanese laid-open patent publication No. Sho 64-55031, a magnetic bearing and a stepping motor have a common magnetic path. This technique is suitable for a low speed actuator, but not for a high speed rotation since it requires a significantly large number of poles. Also, it is difficult to adopt this technique to machines such as a high power induction machine or permanent magnetic type motor with sine-wave shape super magnetic distribution or magnetic distribution.
Japanese laid-open patent publication No. Hei 4-26188 discloses a machine having a lesser number of poles and having a structure similar to that of a conventional induction machine or permanent magnet type rotary machine.
The machine includes a stator with eight teeth similar to the stator core of a four-phase switched reluctance machine. Four-pole concentrated windings are wound around the stator and divided by respective magnetic poles. The magnetic poles have independently controllable magnetic fluxes.
It is also possible to form a rotating magnetic field and to produce a radial force with fluctuation of magnetic flux of each of the magnetic poles. Japanese laid-open patent publication No. Hei 4-107318 discloses a similar core having distribution windings and providing super magnetic distribution similar to sine-wave distribution. In these technique, however, one unit, which produces two cross radial forces and a torque, requires a minimum of eight single-phase inverters and sixteen wires if two-phase winding are used. This is because four divided windings are independently driven. It also requires a high speed, high accurate, high capacity current drive unit since the radial force and torque are controlled by the same winding current.
To this end, the applicant of the present application has previously proposed an electromagnetic rotary machine wherein two-pole windings are mounted to a four-pole rotary machine so as to produce a radial force, as disclosed in Japanese laid-open patent publication No. Hei 2-193547. This electromagnetic rotary machine is a rotating magnetic field type motor which includes position control windings mounted to a stator. The position control windings are different in pole number and produce a torque as well as a radial force by positively making an imbalanced rotating magnetic field.
FIG. 20 shows the principle of a radial force produced by this type of electromagnetic rotary machine.
A stator 42 includes four-pole windings 44 adapted to produce a torque. A positive current flows through the four-pole windings 44 to form four-pole symmetrical magnetic fluxes H4 when a rotor 40 is located centrally within the stator 42. Four-pole windings, not shown, extend at right angles to the four-pole windings 44. A two-phase alternating current is caused to flow through the four-pole windings so as to form four-pole rotating magnetic fields. As indicated earlier, they may be replaced by three-phase windings. A torque is applied to the rotor 40 if cage windings are mounted to the rotor 40 to form a cage induction machine.
The stator 42 additionally includes two-pole position control windings 46a and 46b adapted to exert a radial magnetic force on the rotor 40. As shown in FIG. 20, two-pole magnetic fluxes H2 are developed when a positive current flows through the position control windings 46a.
In this case, the direction of the magnetic flux H4 by the four-pole windings 44 is opposite to the direction of the magnetic flux H2 by the two-pole windings 46a at the lower gap of the rotor 40 as shown in FIG. 20. As such, the density of the magnetic flux decreases at this gap. On the other hand, the direction of the magnetic flux H4 of the four-pole windings is identical to the direction of the magnetic flux H2 of the two-pole windings at the upper gap of the rotor 40. As such, the density of the magnetic flux increases at the upper gap.
Such an imbalanced magnetic flux distribution causes an upward radial force F, see FIG. 20, to be exerted on the rotor 40. The extent of the radial force F varies with the magnitude of a current flowing through the two-pole windings 46a. A downward radial force can be produced when a current flows through the two-pole windings 46a in an opposite direction.
To produce a lateral force in FIG. 20, a current is caused to flow through the two-pole windings 46b which extend at right angles to the windings 46a. It is thus possible to change the extent and direction of a force by selecting the magnitude and direction of a current flowing through the two-pole cross windings 46a and 46b.
Referring to FIG. 20, the four-pole windings 44 are used to drive the motor, and the two-pole windings 46a and 46b are used to control radial position of the rotor.
Alternatively, the four-pole windings 44 may be used to produce a radial force, and the two-pole windings 46a and 46b may be used to drive the motor.
To produce a radial force and a torque, this electromagnetic rotary machine only requires six three-phase windings and two three-phase inverters. Also, the windings which produce a radial force are separated from the windings which produce a torque. This allows for the use of a small radial force control inverter or amplifier. As the electromagnetic rotary machine employs four-pole and two-pole windings, mutual coupling is zero when the rotor is located centrally within the stator. This prevents application of an induction voltage to the radial force control windings. This principle is widely applicable to a high power rotary machine such as an induction machine, a permanent magnet type synchronizer, and a synchronous reluctance motor with sine-wave magnetic force distribution or sine-wave magnetic flux distribution.
A multiplicity of serial conductors and parallel conductors are used in the rotor of a large winding-type induction machine. These conductors have more slots and provide sine-wave distribution. As the rotor is rotated at high speeds, it must have such a strength as to resist a centrifugal force.
To this end, a high speed rotary machine typically includes a cage rotor with a main shaft being supported by a magnetic bearing. The cage rotor is made of metal which is high in mechanical strength and electrical conductivity. This rotor has a small secondary resistance and is durable.
However, a two-pole induction current flows in the cage rotor if two-pole rotating magnetic fields are developed in the stator of the electromagnetic rotary machine. Also if four stator windings develop four-pole rotating magnetic fields, a four-pole induction current flows through rotor conductors. That is, the cage rotor can be two-pole or four-pole windings with respect to the stator. Accordingly, mutual inductance occurs between the four-pole and two-pole stator windings and the cage windings.
As a result, a two-pole winding current substantially flows toward the cage windings if the two-pole windings are used, for example, as position control windings. This results in generation or less of heat or phase delay. The same problem occurs when the four windings are used as position control windings.
The phase delay can be overcome by phase advancement compensation. The amount of phase delay can be obtained theoretically by measuring motor constant. In a magnetic floating system, displacement of an object is generally measured to effect feedback control. In order to stabilize the system, it is necessary to effect differential control. As such, if phase delay or lag occurs, phase lead angle to be maintained by differential control increases.